1. Field of the Invention
The present invention relates generally to a method for manufacturing a low loss optical fiber, and more particularly, methods for producing an optical fiber doped with an alkali metal oxide.
2. Technical Background
Attenuation is a principal limiting attribute of optical fibers. Optical fiber loss, for example, plays an important role in setting the limiting distance between optical fiber amplifiers. This is particularly important in long distance and ultra-long distance networks such as, for example, undersea applications, where such amplifiers represent a significant system cost, as well as a major factor in system reliability. Consequently there is a tremendous amount of commercial interest in reducing attenuation to the lowest possible level.
For silica-based optical fibers used in long distance telecommunication transmission networks, attenuation losses have been reduced to the point where most of the remaining attenuation is due to intrinsic scattering within the glass material. It is generally accepted that intrinsic scattering is a combination of losses associated with density and dopant concentration fluctuations. Density fluctuations are closely proportional to the glass transition temperature, Tg, defined as the temperature at which the melt viscosity is 1013 poise, and may result in both large and small-angle scattering losses.
One means of lowering the Tg, and therefore the attenuation, is to add a modifier to the core glass. Such modifiers, if chosen appropriately, are capable of significantly reducing scattering losses in the fiber core, and therefore the attenuation of the fiber. Alkali metal oxides serve as efficient modifiers—a concentration of approximately 0.5 mole percent of an alkali metal oxide can reduce the attenuation of silica glass by as much as 25%.
The most common, commercially available optical fibers are SiO2-based, and the theoretical lower limit for the attenuation in such fibers is generally accepted to be about 0.15 dB/km. It is known in the art that some non-SiO2 glasses, such as high-alkali alumino-silicate and fluoride glasses, are capable of achieving losses lower than SiO2-based fiber, however these approaches have not yet been commercially realized in long-length transmission fiber.
Silica-based glasses containing an alkali metal oxide dopant, alone or in combination with other compounds, such as, for example, CaO, Al2O3 or F, have been proposed as core materials for optical fibers having intrinsic scattering losses lower than that of pure vitreous SiO2. However, attempts to manufacture such fibers have resulted in attenuation levels much higher than the theoretical lower limit. In the case of the multi-dopant glasses, increased dopant concentration fluctuations and crystallization have proven difficult to overcome. For both single and multi-dopant glasses, high levels of contaminants, such as, for example, transition metals and −OH, have made it difficult to achieve the desired low attenuation. Often these contaminants are unintentionally introduced during the doping process.
Conventional soot-to-glass fiber making processes, such as outside vapor deposition (OVD) and vapor axial deposition (VAD), are not well suited to alkali metal oxide doping. One reason for this unsuitability is the unavailability of simple, easy-to-deploy high vapor pressure alkali metal source compounds. In addition, the soot preforms that result from these processes generally contain H2O, a combustion by-product generated during the soot laydown process. This H2O can disassociate during further processing of the soot preform to form −OH. −OH can have a deleterious effect on fiber attenuation, particularly when present in the core of the fiber. Typically, this −OH is removed by flowing chlorine through the preform at an elevated temperature. Unfortunately, this drying step would likewise remove any alkali metal oxide that would be deposited in an OVD or VAD process by forming an alkali chloride. Moreover, any alkali chloride that remained in the preform after the chlorine drying step would form alkali chloride crystals upon cooling. Such crystals cause the glass to become opaque, making it unsuitable for the transmission of light. Further, the alkali metals also increase the crystallization rate of the silica itself such that the alkali metal oxide-doped silica soot that would be deposited in an OVD or VAD process would tend also to crystallize before it can be sintered into dense, defect-free glass.
One technique for incorporating an alkali metal oxide into silica glass is by diffusing an alkali metal directly into consolidated glass. However, efforts to diffuse alkali metals into silica glass have suffered from the simultaneous diffusion of impurities, including transition metals and water, resulting in losses well above the theoretical minimum. It would be desirable to develop a method of doping a silica glass optical fiber precursor with an alkali metal such that a fiber drawn therefrom would have a low optical loss.